This invention relates to improvements in a drive circuit for a gate turnoff thyristor.
The gate turnoff thyristor (hereinbelow, termed "GTO") is an element of the so-called self-extinguishing type having a gate electrode in which a main current can be turned "off" by causing a negative reverse current to flow to the gate electrode. Since it does not require a commutation circuit, it has the advantage that devices can be made smaller in size. It has therefore come into extensive use in chopper circuits, various inverter devices, etc. However, in order to interrupt the main current (anode current) of the GTO by means of the gate electrode, the negative reverse gate current needs to rise quickly, to exhibit the longest possible attenuation time and to have a negative peak value large enough to match an anode turnoff current.
FIG. 1 is a diagram showing a typical example of a conventional drive circuit for a GTO. First, the arrangement of this circuit will be explained. Referring to the figure, numeral 2 designates the drive circuit for driving the GTO 1. A switching element capable of turning "on" and "off", such an n-p-n transistor 4, a resistor 5, and a capacitor 6, are connected in series in this order between the anode of a D.C. power source 3 and the gate electrode G of the GTO 1. The cathode of the D.C. power source 3 is connected to the cathode K of the GTO 1. In addition, a resistor 7 and a diode 8 are connected in series, and this series circuit is connected in parallel with the capacitor 6. Further, a reactor 10 is connected in series with a thyristor 9 for discharging current from the capacitor 6 as the reverse gate current of the GTO 1 between the cathode K of the GTO 1 and the node of the capacitor 6 and the resistor 7. Symbol A indicates the anode of the GTO 1.
FIG. 2 is a diagram showing the waveforms at various portions for explaining the operation of the circuit in FIG. 1.
Next, the operation of this circuit will be explained with reference to FIG. 2. Now, under the "off" state of the thyristor 9, a base current I.sub.b as shown at (a) in FIG. 2 is produced at a time t.sub.1 to turn the n-p-n transistor 4 "on". Then, a pulse current i.sub.GM which rises sharply and has a great peak value as shown at (c) in FIG. 2 flows from the D.C. power source 3 through the resistor 5 as well as the capacitor 6. Subsequently, a forward gate current i.sub.G necessary for holding this ignition of the GTO 1 as shown at (d) in FIG. 2 flows through the resistor 7 as well as the diode 8. That is, the so-called high gate current (i.sub.GM +i.sub.G) having an abrupt leading edge and a sufficient trailing edge as shown at (f) in FIG. 2 flows in correspondence with the turnon period of the n-p-n transistor 4, and the GTO 1 is ignited. Next, at a time t.sub.2, the n-p-n transistor 4 is turned "off", and the thyristor 9 is simultaneously ignited by causing a gate current I.sub.G to flow as shown at (b) in FIG. 2. Then, the charge with the illustrated polarity and stored in the capacitor 6 due to the pulse current i.sub.GM is discharged through the reactor 10, and a reverse gate current i.sub.GR as shown at (e) in FIG. 2 flows from the cathode K toward the gate electrode G of the GTO 1 until the main current (anode current) of the GTO 1 is interrupted. The operation during this time interval will be describd in more detail. For a while since the reverse gate current i.sub.GR has begun to flow through the GTO 1, the path between the cathode K and gate electrode G of the GTO 1 is in a nearly short-circuited state on account of stored charge, and the reverse gate current i.sub.GR of the GTO 1 increases at the rate (-di.sub.GR /dt) which is substantially determined by the inductance of the reactor 10 and the charged voltage of the capacitor 6. However, when the junction between the cathode K and the gate electrode G has recovered after a period t.sub.s (storage time), an avalanche voltage develops across the cathode K and the gate electrode G, and the reverse gate current i.sub.GR attenuates suddenly. As stated before, in order to interrupt the anode current of the GTO 1 by means of the gate, the attenuation time of the reverse gate current i.sub.GR needs to be held sufficient long (at least, equal to the trailing time of the GTO 1). The reactor 10 is therefore inserted even when the rise (-d.sub.GR /dt) of the reverse gate current i.sub.GR is somewhat sacrificed.
With such a drive circuit for the GTO 1, the rise rate of the reverse gate current i.sub.GR is sacrificed for the attenuation time thereof to be ensured. For this reason, the storage time (t.sub.s) in the gate turnoff mode lengthens, which has led to the problem that, not only the operating performance of a device using the GTO 1 is deteriorated, but also the GTO 1 itself is occasionally destroyed because a current concentration period is excessive during the transient process of the turnoff. Moreover, since the two switching elements formed of the n-p-n transistor 4 for causing the forward gate current i.sub.G to flow and the thyristor 9 for causing the reverse gate current i.sub.GR to flow need to be alternately driven and controlled so as to be operationally opposite with respect to each other, there has been the problem that a circuit for the control becomes complicated.